Preliminary Tests; Dilution - wtw photolab S12 Operating Instructions Manual

photoLab ® Series
3 Sample Preparation

3.2 Preliminary Tests

Correct measurement results can be obtained only within the measuring
range spe ci fied for each indi vi dual parameter. When dealing with sample
solutions of an un known concentration, it is advisable to establish whether
the sample concentration is indeed within the specified measuring range,
ideally roughly in the middle of the range.
Preliminary tests enhance the analytical reliability and make the determi-
nation of the necessary dilution ratios in the case of high concentrations
®
easier. Merckoquant Test Strips are very well suited for preliminary
tests.

3.3 Dilution

Dilution of samples is necessary for two reasons:
•
The concentration of the parameter under investigation is too high, i. e.
it lies out side the measuring range.
•
Other substances contained in the sample interfere with the determina-
tion (matrix interference); false-high or false-low results may ensue.
The following auxiliaries are absolute prerequisites for the dilution of the
sample:
•
Volumetric flasks of varying sizes (e.g. 50, 100 and 200 ml)
•
Positive-displacement pipette
•
Distilled or DI water.
Only dilutions carried out with these auxiliary pro ducts are of sufficient reli-
ability in the area of trace analysis, to which photometry belongs (for the
sim pli fied procedure see page 11).
An important aspect here is that once the volumetric flask has been filled
up to the mark with distilled water the flask is closed and the contents are
thoroughly mixed.
The dilution factor (D
F
as follows:
Final volume (total volume)
D =
F
Initial volume (sample volume)
The analytical result is subsequently multiplied by the dilution factor.
A calculation can be dispensed with when the dilu tion is programmed into
the pho tometer. The dilution number (see the table on page 11) is
entered and the measure ment value is subsequently calculated cor rectly
and immediately displayed.
10
) resulting from the dilution procedure is calculated
Release 01/2012